Lurking within the arterial wall of many seemingly healthy persons are one or more pools of lipid, referred to as vulnerable plaques. A fibrous cap covers the vulnerable plaque and separates its contents from the blood flowing in the lumen of the artery. Should this cap rupture, thrombogenic tissue will be exposed. This causes a cascade of blood clotting and thrombus formation that may occlude the artery. Depending on where this occurs, the result can be a sudden heart attack or stroke.
Because of the potentially devastating consequences of a vulnerable plaque, it is desirable to locate such plaques before the rupture of the fibrous cap. One method of doing so is to illuminate the wall of the artery with infrared light at a range of wavelengths and collecting the light reflected at each wavelength. If a vulnerable plaque is present, the spectrum obtained from the collected light will bear a characteristic signature.
There is, however, no guarantee that the collected light will have been reflected by structures within the arterial wall. In fact, a significant portion of the collected light is reflected not from within the arterial wall but from the blood or external to the wall itself. It is therefore desirable to discriminate between light reflected from within the arterial wall and all other light.